It shall be ensured for adequate anesthesia that the patient will not remember the procedure, the responses to painful stimuli are suppressed and circulation and respiration are maintained despite the highly potent drugs necessary for this, and the oxygen supply of the brain and other organs is thus guaranteed at any time. It shall be ensured for this that the drugs used for the anesthesia are present at a reasonable concentration at their sites of action, e.g., in the brain in the case of centrally acting anesthetics, such as sedatives or centrally acting analgesics, such as opiates, or at the motor end plates between motor nerves and skeletal muscles in the case of paralyzing drugs, e.g., muscle relaxants. The necessary concentration may differ depending on the situation, e.g., due to the surgical procedures being performed. For example, the pain stimulus elicited by a skin incision in the upper abdominal region may be markedly weaker than the pain stimulus caused by incisions for resecting a kidney.
If the need for anesthetic action changes in the course of, e.g., a surgery, an adequate anesthesia shall ensure that this need is met as quickly as possible.
However, adequate anesthesia is characterized at the same time by the anesthetic drugs not being dispensed at excessively high doses, in order to, among other things:
minimize adverse side effects of the drugs used for the anesthesia, such as a secondary weakening of the intensity of contraction of the heart muscle or respiratory depression; and
prevent avoidably high concentrations of anesthetic active ingredients from accumulating in endogenous storage sites or compartments such as fatty tissue, which excessively delay the desired end of an anesthesia due to reflooding.
Achieving adequate anesthesia is especially challenging for an anesthesiologist, among other things, because the concentrations of anesthetic active ingredients in the blood or at the site of action and the anesthetic effects resulting therefrom, such as analgesia, sedation and paralysis, are unknown to the anesthesiologist or are known to the anesthesiologist to a greatly limited extent only. This is especially true of drugs that are administered intravenously (i.v.). The situation is further aggravated by the fact that more than one anesthetic drug is often administered, and that especially the analgesics (e.g., opiates) and sedatives (e.g., propofol) synergistically interact with each other. For example, the degree of increase in the effect of propofol following an increase in the concentration of propofol at the site of action depends on an opiate concentration that is likewise present at the site of action.
Technical solutions, which can facilitate the control of an adequate anesthesia for the anesthesiologist, are known from the state of the art:
a. The effect of muscle relaxants can be measured directly, even though only on selected skeletal muscles, and not always on skeletal muscles that are relevant for the particular procedure (e.g., the thumb muscle).
b. The concentration of volatile anesthetic drugs can be measured during expiration in the breathing gas. The end-expiratory concentration thus measured reflects the concentration in the lungs. However, it reflects the concentration in the blood plasma to a limited extent only, and that in other tissues or at the site of action to an even more limited extent only.
c. The monitoring of electrical voltages between certain sites on the scalp (EEG) makes it possible to infer the electrical activity in the brain. Devices are known, by means of which these voltage signals can be interpreted by means of very demanding processes of digital signal processing and aggregated into a standardized value, which is in a—not always close—relationship with the depth of sedation of the patient. One example of such a device is the BIS™ monitor from the firm of Aspect Medical.
d. Processes are known by means of which the concentration curves of anesthetic drugs can be determined and predicted on the basis of models based on the knowledge of the quantities of drugs introduced into the patient intravenously or via the lungs. These models have been determined based on measurements on a plurality of patients and permit a certain adaptation to the patients actually involved by stating the age, body weight, etc., of the patient to be actually treated. These processes fall in the area of pharmacokinetics.
e. Furthermore, processes are known by means of which the anesthetic effects of the drugs being used can be determined from the quantities of active ingredient administered. These effects are frequently represented as a probability that a patient responds to a given stimulus (e.g., a skin incision or being spoken to/shaken). These probabilities are based on statistical models, which have been determined on the basis of measurements on a plurality of patients. These models can also describe the mutual interaction of a plurality of anesthetic drugs. These processes fall in the area of pharmacodynamics.
f. Furthermore, processes for the graphic display of pharmacokinetically and pharmacodynamically determined concentration and effect curves are known. These include displays on the time axis (time-based display) as well as displays of opiate concentrations versus sedative concentrations (concentration-based display as is described in Patent DE 10 2004 050 717 B3 and corresponding US2006081244—U.S. patent application Ser. No. 11/250,026 of Oct. 13, 2005, which is hereby incorporated by reference).
Besides the adequate anesthesia, as it was described above, the anesthesiologist also has the task of monitoring and possibly securing bodily functions, which are limited or threatened based on side effects of the anesthetic drugs, based on therapeutic or surgical procedures, or based on a disease of the patient. These bodily functions include, e.g., the blood pressure, body temperature and respiration.
A number of possibilities of procedures, e.g., hemodynamically active drugs, volume replacement (e.g., physiological sodium chloride solution), heating or cooling techniques and respirators, are available to the anesthesiologist to perform these tasks. To make possible the adequate control of the use of these possibilities of procedure, various measurement methods, by means of which relevant physiological variables or surrogates thereof can be measured, are available to the anesthesiologist. These include, e.g., the end-expiratory CO2 concentration measurement, invasive or non-invasive blood pressure measurement, heart rate measurement, etc.